Membrane bioreactor (MBR) technology is a highly efficient wastewater treatment and reclamation technology which represents an organic combination of a membrane separation technique and a conventional biological treatment technique. In a MBR system, various contaminants in wastewater are removed mainly by biological actions of microorganism. However, differing from the conventional biological treatment technology, membrane modules replace a secondary sedimentation tank and thoroughly separate sludge from water with highly efficient separation function, such that sludge retention time and hydraulic retention time may be independent from each other, which leads to increased concentration of active sludge in a biological reaction tank and accumulation of special bacteria in active sludge and thus enhances biological reaction rate. Separation of microorganism from water is no longer achieved through gravity sedimentation, instead, water molecules and some other small molecule substances permeate the membrane under pressure while microorganism and large molecule substances are intercepted in the biological reaction tank by the membrane. Therefore, excellent effluent quality is obtained by the system. The MBR technology substantially solves some prominent problems commonly existing in conventional wastewater treatment technology, for example, unstable effluent quality, large occupied area and complicated processing control, etc., and is a water treatment technology with great development potentiality. Especially in wastewater reclamation and reuse, domestic wastewater, municipal wastewater or similar industrial wastewater may be treated through MBR technology in one step into high quality reclaimed water to be used as municipal miscellaneous water, industrial circulating cooling water, etc. Therefore, MBR technology receives a widespread academic attention day by day all over the world, and large scale engineering application thereof is also increasing.
According to the position where the membrane module is disposed, membrane bioreactors may be divided into two major classes, i.e., separate, namely side stream, membrane bioreactor and integrated, namely submerged, membrane bioreactor. In general, a separate membrane bioreactor largely selects a columnar hollow fiber membrane module or a tubular membrane module, in which the membrane module and the bioreactor is disposed separately, mixed fluid in the bioreactor is delivered to the feed entrance of the membrane module after being pressurized by a circulating pump, liquid in the mixed fluid permeates the membrane under pressure and becomes treated effluent of the system, while solids, large molecule substances and the like are intercepted by the membrane and flow back into the bioreactor with concentrate. An integrated membrane bioreactor largely selects a curtain type or bundle type hollow fiber membrane module or a plate-frame type membrane module, in which the membrane module is submerged below the fluid level of the bioreactor. After raw water enters the membrane bioreactor, the majority of contaminants are decomposed or transformed by active sludge in the mixed fluid, and then water is filtered and discharged through the membrane under the negative pressure provided by a suction pump or under level difference. An aeration system is provided under the membrane module, and thus on one hand, provides microorganism with oxygen necessary for decomposing organic compounds, and on the other hand, based on gas stripping principle, enables the air-water two-phase flow to hydraulically flush the outer surface of the membrane, so as to restrain cake layer sediment on the membrane surface.
In table 2, part of the patents currently available domestically and abroad with respect to structural design on the hollow fiber membrane module are listed.
TABLE 2Part of the patents with respect to structuraldesign on the hollow fiber membrane moduleCN patent No. orpatent application No.Applicant or Patentee02158708.6, 02158711.6Institute of Membrane Science andTechnology, Tianjin Polytechnic University03822704.5Mitsubishi Rayon Co., Ltd., Japan200420029337.7Tianjin Haiyue Water Treatment Hi-Tech Co.,Ltd.200420109650.1,Zhejiang Omex Environmental Engineering200510049324.5Co., Ltd.2005800225651.5Koch Membrane Systems Inc., Germany200610065807.9Minliang Zhang200620039621.1Shanghai Dehong Biology Medicine Science& Tech Development Co., Ltd.200710056641.9Tianjin Motimo Membrane Technology Co.,Ltd.
These patents or patent applications have the following problems: the phenomenon that sludge accumulates at the roots of the hollow fiber membrane filaments is serious, and once sludge accumulation is formed, it is not easy to remove, which decreases effective filtration area and filtration uniformity of the hollow fiber membrane module. Continuous development and overspreading of membrane fouling at the roots of the hollow fiber membrane filaments in turn further increase uneven filtration and the loss of effective filtration area, and eventually shorten the life of the hollow fiber membrane module. Reference is made to FIG. 1, which is a schematic view of purging at a lower end part of a hollow fiber membrane module in the prior art. A short pipe 2 is previously embedded in the lower end part 1 of the hollow fiber membrane module, and gas is aerated through the short pipe 2 to enhance purging effect to the hollow fiber membrane filament 3. However, the orifice of this short pipe is opened upward, and the upper edge of the orifice is higher than the cast end surface of the lower end part 1 of the hollow fiber membrane module by a certain height, and aerated air flow thus generated, as shown in FIG. 1, is not able to purge and flush thoroughly the root of the hollow fiber membrane filament. Therefore, the problem that sludge accumulates at the roots of the hollow fiber membrane filaments is still not well solved.